JP 2013-72342 A discloses a control device for an internal combustion engine which is configured to control an engine in which a part of exhaust gas as external EGR gas is recirculated from an exhaust system to an intake system. In such a conventional control device, an opening degree of an EGR valve is controlled based on a map defining a relationship between an operating range defined by engine speed and engine load and a target amount of external EGR gas (hereinafter referred to as a “target EGR amount”). In the map, the operating ranges are partitioned by contour lines of the target EGR amount. According to the map, the target EGR amount is set to a highest value in a partitioned range including a middle-engine-speed-and-middle-engine-load range, and decreases from this partitioned range toward a peripheral partitioned range.
The target EGR amounts in the map are obtained by an experiment or simulation performed in advance. According to the map, an actual external EGR amount (hereinafter also referred to as an “actual EGR amount”) can be maintained at an optimum value during a steady operation in which the engine operating state stays in a partitioned range having an equal target EGR amount. On the other hand, the actual EGR amount is largely affected by time lag during a transition operation in which the engine operating state is transferred across the contour line of the target EGR amount. When the engine operating state is transferred from a partitioned range with low target EGR amount to a partitioned range with high target EGR amount, for example, the large influence by time lag causes a period during which the actual EGR amount does not achieve the target EGR amount, resulting in knocking easily occurring in the period. Accordingly, a countermeasure against such knocking during the transition operation is needed.
The present disclosure addresses the above described problem, and an object of the present disclosure is to take a knocking countermeasure when the engine operating state is transferred from the partitioned range with low target amount to the partitioned range with high target amount, in a case where an opening degree of an EGR valve is controlled based on the map defining a relationship between the target amount of external EGR gas and the engine operating range.
A first aspect of the present disclosure is a control device for an internal combustion engine which is configured to control an engine in which a part of exhaust gas as external EGR gas is recirculated from an exhaust system to an intake system,                wherein the control device comprising:        an EGR map the defines a relationship between an operating range defined by engine speed and engine load and a target value of external EGR rate, and has a predetermined partitioned range in which the target value is set to a highest value; and        an operating angle map that defines a relationship between the operating range and an operating angle of an intake cam for driving an intake valve of the engine,        wherein the operating angle map being set so that        a large operating angle is selected in a first region including a region corresponding to the predetermined partitioned range, the large operating angle being capable of closing the intake valve in a first crank angle section including a crank angle at which a suction efficiency becomes highest, and        a small operating angle is selected in a second region in which the engine load is higher than that of the first region, the small operating angle being capable of closing the intake valve in a second crank angle section that is located nearer to a bottom dead center side than the first crank angle section,        wherein the control device is configured to:        select the operating angle in accordance with the operating angle map when it is predicted that the engine operating state stays in a partitioned range having the equal target value in the EGR map; and        when it is predicted that the engine operating state is transferred from a partitioned range with the low target value to a partitioned range with the high target value in the EGR map, change a boundary between the first region and the second region in a direction of decreasing the engine load, and then select the operating angle in accordance with the operating angle map.        
A second aspect of the present disclosure is the control device for an internal combustion engine according to the first aspect,                wherein the control device is further configured to:        when it is predicted that the engine operating state is transferred from a partitioned range with the low target value to a partitioned range with the high target value in the EGR map,        when the engine operating state is transferred in a direction of increasing the engine speed and the engine load, increase a degree of a change of the boundary as a positive change rate of an accelerator opening degree of the engine becomes larger; and        when the engine operating state is transferred in a direction of decreasing the engine speed and the engine load, increase the degree of the change of the boundary as a negative change rate of the accelerator opening degree becomes larger.        
A third aspect of the present disclosure is the control device for an internal combustion engine according to the first aspect,                wherein the control device is further configured to:        when it is predicted that the engine operating state is transferred from a partitioned range with the low target value to a partitioned range with the high target value in the EGR map,        calculate a time interval from a change point of the target value set in accordance with the EGR map to an increase starting point of an actual external EGR rate; and        increase the degree of the change of the boundary as the time interval is larger.        
A fourth aspect of the present disclosure is the control device for an internal combustion engine according to any one of the first to third aspects,                wherein the engine comprising a turbocharger including a compressor and a turbine, and        the external EGR gas is recirculated from a downstream side of the turbine to an upstream side of the compressor.        
According to the first aspect, when it is predicted that the engine operating state is transferred from a partitioned range with low target value of the external EGR rate to a partitioned range with high target value of the external EGR rate in the EGR map, the boundary between the first region and the second region is changed in a direction of decreasing the engine load, and then the operating angle can be selected in accordance with the operating angle map. Thus, when the target value of the external EGR rate increases, the small operating angle can be selected in a period during which the actual external EGR rate does not achieve the target value. The intake valve can be closed in the second crank angle section by selecting the small operating angle, thereby lowering the suction efficiency as compared to a case where the large operating angle is selected to close the intake valve in the first crank angle section including a crank angle at which the suction efficiency becomes highest. Accordingly, an in-cylinder condition can be improved in the period during which the actual EGR rate does not achieve the target EGR rate, to avoid a state liable to cause knocking. Since the second crank angle section is located nearer to the bottom dead center side than the first crank angle section, the gas amount returned to an intake port from a cylinder while the intake valve is opened can be reduced, thereby suppressing a temperature increase of in-cylinder gas. Even when the suction efficiency is not lowered as compared to a case where the large operating angle is selected regardless of selecting the small operating angle, the state liable to cause knocking can be avoided.
According to the second aspect, the degree of the change of the boundary between the first region and the second region can be changed in accordance with the change rate of the accelerator opening degree. Accordingly, the in-cylinder condition in the period during which the actual external EGR rate does not achieve the target external EGR rate is improved in accordance with the degree of the influence by the time lag, to avoid a state liable to cause knocking.
According to the third aspect, the time lag from the change point of the target value of the external EGR rate set in accordance with the EGR map to the increase starting point of the actual value of the external EGR rate is directly calculated, and the degree of the change of the boundary between the first region and the second region can be increased as the time lag is larger. Therefore, the in-cylinder condition can be improved in the period during which the actual external EGR rate does not achieve the target value of the external EGR rate, to avoid a state liable to cause knocking.
According to the fourth aspect, an in-cylinder condition of the engine provided with an LPL-EGR device can be improved in the period during which the actual external EGR rate does not achieve the target value of the external EGR rate, to avoid a state liable to cause knocking.